This invention relates generally to propellant compositions and more particularly to solventless double base propellants having an energetic plasticizer with no secondary nitroxy groups.
Double base propellants are homogenous propellants having a binder e.g. nitrocellulose and an energetic plasticizer. The use of these propellants in gun, missile or gas generating auxiliary equipment is known in the art. This type of propellant has many advantages over single base propellants. For example, single base propellants have a greater variance in performance. This is due to their having been manufactured by a solvent process and retaining a varying amount of residual volatile solvent. A solvent process is needed to manufacture a single base propellant. As the amount of solvent is increased, an objectionably long drying cycle becomes required by the solvent process, and the amount of retained residual solvent also increases. However the adoption of solventless double base propellants for gun and auxiliary equipment uses has been slight due mainly to a lack of an alternative energetic plasticizer to nitroglycerin.
Although nitroglycerin is extremely energetic and has a good plasticizing capacity, the many disadvantages of this nitrate ester discourages its use. First of all, nitroglycerin is extremely hazardous. The sensitivity and high energy make nitroglycerin dangerous to handle and makes the resulting propellant composition more sensitive to unwanted detonation. Also nitroglycerin is volatile and the resulting vapors cause sickness and headaches to humans, thereby causing health problems in the manufacture, handling, and storage of any composition containing nitroglycerin.
The sensitivity of nitroglycerin also causes the propellant composition to have a high hazard classification which means extra expense for storage, and a more limited reserve. Another disadvantage is exudation. Nitroglycerin has a tendency to migrate out of the composition, and thereby result in poorer firing accuracy due to variance in propellant strength.
Flame temperatures of a propellant containing nitroglycerin are high. This characteristic necessitates the addition of coolants which produce soot and smoke in the exhaust. If coolants are not used, the high burning temperature excessively erodes the barrel in comparison with single-base powders.
In applications requiring a high degree of mechanical strength e.g. most missile and many gun propellant uses, the solvent process is used to make the double base propellants. However, difficulties are encountered in the process itself and in the end product. The solvent process requires a lengthy drying cycle and a final blending. During the solvent process, volatiles are introduced into the propellant composition. Volatiles shorten the shelf life of a propellant. With nitroglycerin propellants, a serious problem is compounded in that compositions containing nitroglycerin have already a poor shelf life as compared to single base propellants.
Since the beginning of World War II much effort has been expended to find an alternative plasticizer to nitroglycerin and a way of avoiding the necessity of a solvent manufacturing process for most gun propellants. Unfortunately the resulting compositions had poor performance or poor mechanical strength or had to rely excessively on solvents in order to process the propellant.
Nitrate esters like polyolpolynitrates have excellent energy content. But upon contact with ordinary nitrocellulose, only the outer layers of the nitrocellulose softened. No plasticizing of the nitrocellulose takes place, which is essential for any propellant composition. Other nitrate esters are good plasticizers but have low energy and have many of the drawbacks of nitroglycerin, such as poor stability, high sensitivity, as well as being hazardous to the health of humans.
Some attempts have been made to combine various nitrate esters together or with nitroglycerin. These attempts resulted in a propellant with low energy levels or poor mechanical strength. The method of preparation required the use of solvents or of an expensive preprocessed nitrocellulose or other binder. Since the preprocessing involved solvents, the resulting propellant had many of the drawbacks of a propellant prepared by a solvent process.